Wizard Of Ooze, The (Goosebumps HorrorLand #17) by R.L. Stine

Take a little Horror home with you! Marco is a diehard fan of The Ooze, a comic book villain with the power to sludge his enemies. So when he finds a super-rare edition at the Chiller House gift shop, Marco is sure he has found something special for his collection. Outside of HorrorLand there are a few other people interested in the comic book. The kinds of people you don't want to mess around with. Like a muscular monster of a man dressed as The Ooze. But if he's only wearing a costume, why does he leave an oily mess everywhere he goes? And why is he following Marco?